Global warming, petroleum depletion and energy security have been the main driving forces for the development of renewable fuels that can replace the petroleum-derived fuels, such as gasoline and diesel. Ethanol is currently the most commonly used renewable automobile fuel. It is largely produced by fermentation of sugar- or starch-containing feedstocks, such as cane sugar, corn and wheat. However, the supply of these crops is relatively limited, and many of them can be considered as a human food resource. Another disadvantage is that the production of ethanol from most of these raw materials gives a relatively low net energy gain and a low renewable CO2-efficiency, i.e. the amount of fossil CO2 produced throughout the production chain when producing ethanol from these materials is high. Lignocellulose is a more abundant and less expensive raw material with the potential to give a higher net energy gain.
Lignocellulose is primarily composed of cellulose, hemicellulose and lignin. Cellulose is composed of polysaccharide chains of several hundred to over ten thousand linked glucose units, whereas hemicellulose is a polysaccharide composed of xylose, other pentose sugars and various hexose sugars. Cellulose and hemicellulose are tightly associated to lignin, a polyphenolic compound that ties the cellulose and hemicellulose polymers together, thus providing the wood with rigidity and mechanical strength.
In the production of ethanol from lignocellulosic materials, various pretreatment and hydrolysis steps are used to degrade the cellulose and hemicellulose polysaccharides in the lignocellulose to monosaccharides. Microorganisms can then be used to ferment the monosaccharides to ethanol. The yeast Saccharomyces cerevisiae, which metabolizes hexose sugars, is one of the most suitable microorganisms for ethanol production and is favoured in industrial processes.
However, as the lignocellulose is degraded, a broad range of substances are released, some of which can be toxic and inhibit microorganisms, e.g. the yeast S. cerevisiae, which are used for ethanol fermentation. Further, the inhibitory substances may also be rate limiting for enzymes used in the hydrolysis of the lignocellulosic biomass. The nature and amount of inhibitory substances depend on the type of lignocellulosic raw material and the pretreatment and hydrolysis processes used. Examples of inhibitory substances include aliphatic acids such as acetic acid, which is released as the hemicellulose fraction is degraded, furan aldehydes, furfural and different phenolic compounds. The presence of such inhibitory substances thus results in lower ethanol yield and productivity.
Furthermore, it is desirable to recycle the process water in an ethanol production plant, for example to minimize the addition of fresh water and consequently minimize the production costs and/or to reduce the environmental impact. However, recycling of the process water can lead to an accumulation and build-up in the concentration of inhibitory substances, which is an obstacle for reusing the process water. There are several methods to avoid inhibitor-related problems, but they are often associated with additional process cost or other problems.
WO 01/60752 discloses a process for treating the wastewater effluent to decrease the levels of inhibitory substances when producing ethanol from lignocellulose. However, WO 01/6752 involves complex anaerobic digestion with methane producing microorganisms before reusing the wastewater effluent.
Palmqvist et al (Enzyme and Microbial Technology. 1997, 20, p. 286-293) discloses a method for simultaneous enzyme production and detoxification of hemicellulose hydrolysates. However, the method involves detoxification with Trichoderma reesei prior to ethanol fermentation, meaning that hexoses are consumed by T. reesei prior to the fermentation instead of being used for ethanol production, and recirculation of the process water starts prior to fermentation, meaning that only a small portion of the total process water is recirculated.
To summarize, the prior art fails to provide an attractive method for producing ethanol that is simple and allows for detoxification and recirculation of the process water.